As separators for secondary batteries including lithium secondary batteries, porous polymer films have been used hitherto which have excellent permeability to electrolyte solutions. However, due to the porous nature of such a porous polymer film separator, use thereof as the separator in a secondary battery has inevitably had a practical problem that as the battery is repeatedly charged and discharged, dendrites grow and penetrate through pores of the porous film to cause a short-circuit or the dendrites fall from the electrode to cause a decrease in capacity.
JP-A-53-84134 discloses a separator for secondary batteries, such as lithium secondary batteries, which is obtained by grafting an electrolyte solution-permeable polymer onto a porous substrate thereby to fill the pores of the substrate with the polymer. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) However, this separator film is not suited for practical use because it swells too much. Although a measure for controlling the swelling of the film is disclosed in JP-B-62-17822, this technique has failed to provide a substantial solution to the swelling problem. (The term "JP-B", as used herein, means an "examined Japanese patent publication".)
JP-A-2-82457 discloses a method of producing a separator which comprises impregnating a porous resin film with a monomer and then irradiating the resulting film with ultraviolet rays to polymerize the monomer, thereby to fill the pores of the film with the polymer. However, this method has a drawback that the adhesion between the porous resin and the polymer is so poor that the separator develops voids at the interface between the porous resin and the polymer. Thus, this method has been unable to solve the problem of dendritic growth.
Further, JP-A-3-41107 discloses a separator modification utilizing plasma-initiated polymerization. In general, plasma-initiated polymerization is a technique of polymerizing a gaseous-phase or liquid-phase monomer by irradiating the monomer with a plasma (as described in "Plasma Polymerization" written by Yoshihito Osada et al., published by Tokyo Kagaku Dohjin, Japan). This method, however, has also failed to improve the adhesion between the polymer substrate and the polymer formed by plasma polymerization to a desired level. In addition, even in the case where the polymerization of a monomer was conducted by irradiating a substrate with a plasma to graft-polymerize the monomer onto the substrate, the thus-obtained separator was unable to sufficiently inhibit dendritic growth.